Method of and apparatus for indicating the speed of electric motors.



E. F. W. ALEXANDER-S01 METHOD OPAND APPARATUS FOR INDIOATING- TE SPEED 01: ELECTRIC MOTO'YS. APPLIUATIONFILEDSBPT,22,1909.

' .Patentsd. June 191 5.

srarus PATENT oreron.

" 3?. JV. ALEKANDERSGN, OF SCHENECTADY, NEW YORK, ASSIGNOR' T GENERAL ELECTRIC COMPANY, A CORPORATION OF NEVF YORK.

ir'finl'fl-itl lil 03: AND APPARATUS FOR INDICATI NG THE SPEED OF ELECTRIC MOTORS.

Specification of Letters Patent.

Patented June 16, 1975.4.

Application filed September 22, 1909. Serial No. 518,983.

ail who? it may concern known that l, Eurzs'r F. W. Anny on, a citizen of the United "States, g Schenectady, in the county of nests and State of New York, have ited lain new and useful Improvein Methods of and Apparatus for lu- .ho Speed of Electric Motors, of Wll' c r so fl'rliowing is a specification, N ll y invention relates to the measurement no speed of electric motors and of inachines driven by electric motors, and its obfect is to provide a method of: and apparm tus for electrically indicating the speed oi A electric motor, regardless of fluctuations or of variations in load on the motor, i. by means of which the speed of a motor may be accurately determined Without any change in the structure of the motor and Without any apparatus other than a small electrical indicator.

The fundamental equation of an. electric motor having field and an armature is that induced counter electromotive force is proportional to the field flux multipliedby the speed of the armature, and since ii -an l :rnating current motor the fluxat a given ncy is'directly proportional to thle e applied to the field Winding, this ation can he changed, in the case of an ternati'ug current motor, to read, toltage armature terminals voltage acros ..eld tern1ina1s speed a constant. From this equation it is clear what the speed-is prooo oi'ial to the voltage across the armature inals divided by the voltage across the terminals and hence the relative value and of the voltage across the field terminals will he. a correct measure of the speed of the motor, regardless of the voltage imnressed on the motor or of variations in the load assuming the armature resistance to he negligible. In accordance with my in volition, the relative value of the voltage he tween the armature terminals and the vol" lietween bl'l field terminals is indicated suitable instrument which responds z .Jv to the relative value of said voltages is not altccted by their absolute values. e 'orinciple above described can be used ourposes in connection with any otor having field terminals and roinals, but is particularly an voltage and current on the supply cir- 'plicable to alternating cm'reut motors in which the flux in the field varies directly with the voltage impressed ou the field ternnna ls.

Myiuvcntion will best be un ierstood 'in connection with tlii accompanying drawing which illustrates one of the various forms in which it may be embodied and in which-- .l igur'o 1 shows a. portion of a street car equipped with a speed indicator embodying my invention: and Fig. 9 a perspective view of the indicator with some parts hr'r ken away, the motor and its connections lug shown. diagrununatically.

In the specific construction shown in. the drawing, the relation of the parts is best shown in Fig. 2, in which the motor represented as having a field winding 1 and an armature 2, the field winding being provided with field terminals 3, While the armature is provided with armature terminals l. The speed of the motor, and consequently the speed of any device-to Which the motor is positively connected, may be determined bycomparin the voltage across the field terminals 3 with the voltage across the armature terminals 4 and the ratio of one voltage to the other Will accurately show the speed of the motor. The manner in which the ratio of the two volt-ages varies with changes in speed depends upon the extent to which the flux produced by the field Winding is directly proportional to the voltage between the field terminals.

The relative values of the voltage between the field terminals 3 and of the voltage be tween the armature terminals 4- may be detormined in any suitable Way and by vari 'ous forms of instruments, but in the preferred construction the relative values of said voltages are C0] tinuously and accurately determincdby means of a speed. indicator 5 provided with a movable member or shaft 6 on' which is mounted a pointer 7 to cooperate with a scale 8. The rotation of the shaft or member 6 is unrestrained and the shaft is, therefore, free to rotate into any position-where the coils of the instrument may tend to place it. By means of suitable coils or windings a torque dependout upon the voltage between the terminals 3 is exerted on the shaft 6 to turn it in one direction, While at the same time a countertorque dependent on the voltage between the armature terminals d is exerted on the shaft voltages and not at all upon their absolute.

value. Various devices may be used to'exert the opposing torques, but in the preferred construction leads 9 connected to the field terminals 3 conduct current through a voltmeter winding comprising stationary.

coils 10 and movable coils 11 mounted upon the shaft 6, the connections being such that the coils 1'0 and 11 cooperate to produce a torque in. one direction dependent upon the voltage between the terminals 3, while the armature terminals 4 are connected through leads 12 to voltmeter windings consisting of stationary coils 13 and movable coils 14 mounted on the shaft 6 to produce a countertorque which isdependent' upon the voltage between the terminals 4. The various coils are connected, through any suitable connection, such as flexible leads which exert no controlling effect, so that the movable member, formed by the shaft 6 and the coils 11 and 14, is capable of unrestrained rotary movement relatively to the stationary ones and can take up any angular posi- 7 tion.

' The torque developed by coils 11, is a function of the voltage between the ternm nals 3. For any one position of the coils,

the torque is directly proportional to the square of such voltage, and similarly the countertorque developed by coils 14 is for any one position of these coils directly proportional to the square of the voltage across the terminals 4. The-coils Ill and 14 are positioned relatively at an angle to each other on a shaft 6. The torque tending to turn coils 11 is for any one position of these coils equalto V where V is the voltage across the terminals 33 and f, is a certain constant, Similarly, thetorque tending to turn the coils 14 is equal to V where vV is the voltage. across the terminals 44 and 7, is a certain constant. When the indicating pointer 7 comes to rest, the torque tending to turn coils 11 is equal and opposite to the torque tending to turn coils 14. If, for any such position of rest, we represent the I torque tending to turn coils 11 by T, and

the torque tending to turn coils 14 by T the following equations may be obtained:

V vi R of these voltages. Since this is true for any one position, it is true for all positions, and the speed indicator 5 will, therefore, give an indication of the ratio of the voltage across the armature terminals to the voltage across the field terminals, which ratio may be laid out on the scale 8 in units of speed. As the speed of the motor varies, the ratio ofthe voltage between the armature terminals to the voltage between the field terminals also varies in accordance with the equation above stated and since the position of the shaft 6 and the pointer 7 is determined by the relative value of said voltages the pointer 7 will move in accordance with the changes in the ratio of said voltages and the shaft (3 and the pointer 7 will assume positions which accurately indicate the speed of the motor regardless of. variations of voltage on the circuit, since changes in the absolute value of the voltage applied to the voltmeter windings of the indicator produce no change in the position of the shaft 6 as long as the relative value of said voltages remains constant.

My invention is particularly useful in connection with locomotives or cars driven by alternating current motors and by means of it the speed of the car or locomotive may be easily and accurately indicated with no other apparatus than the indicator 5, since all that is necessary is to connect the indicator to the'field terminals and to the armature terminals of the motor. This embodiment of my invention is illustrated in Fig. 1, in which an indicator 5 is mounted on a street car in a position convenient for the motorman and is connected by means of leads 9 and 12 to a alternating current motor 15 which is pos't ively geared to the wheels of the car. Th indicator 5 will accurately show the speed of the car regardless of variations of voltage. on the line, of changes in the profile of the road, or of any other such o no Wish to limit myself to the precise arr, Iige ent disclosed but aim in the appended cl ims to cover all modifications within the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is: 1. The method of measuring the speed of an electric motor having armature terminals and. field terminals which consists in simultaneously measuring and lndicating the ratio .of the voltage'across the armature terminals .1 tutti 15 terminals.

having field terminals and armature termilililS, of a speed indicator comprising two members rigidly secured to a shaft mounted for rotary movement and. means connected to said terminals to exert two opposing lo forces which tend to produce rotation of said menibers in opposite directions, one of said forces being a function of the voltage between the field terminals and the 0th *1- a function of the voltage between the armature l. The combinal ion with an electric motor having tield terminals and armature terminals, of a mo able indicating member, means for exerting on said member a torque 39 llvVElOIXd by and a function of the voltage between said field terminals and means for exerting on said member a counter-torque developed by and a function of the voltage between said armature terminals.

N The combination with an electric motor across the armature terminals for. exerting on said member a countertorque tending to turn it in the opposite direction.

6. The combination With an electric inotor having field terminals and armature terminals, of a speed indicator comprising a pivoted indicating member mounted for retary movement, a voltmeter winding including a coil secured to said indicating member connected across said field terminals and er;- erting a torque on said member, and a second voltn'ieter winding including a second coil secured to said indicating member connected across the armature terminals and. exerting a counter torque on said. member.

7 The combination with an electric motor having field terminals and armature terminals, of a voltmeter winding con'iprising a fixed and a movable coil connected across the field terminals, a second voltmeter Winding comprising a fixed and a movable coil connected across the armature terminals, movable coils of said voltmeters being so related that they tend to move in opposition to one another, and an unrestrained movable member rigidly secured to each or". said movablc coils.

in Witness whereof, I have hereunto set my hand this 20th day of September, 1909, ERNST F W. ALEXAPDEEZSOII. Witnesses BENJAMIN B. HULL, EMMA B. Coons.

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